In the image forming process according to electrophotography, electrostatic latent images are formed on the inorganic photoreceptor such as selenium, selenium alloy, cadmium sulfide and amorphous silicon or on the organic photoreceptor using a charge generator and a charge transporting agent. Then, the images are developed by a toner, transferred to paper, plastic film or the like and fixed to obtain visible images.
As for photoreceptors, depending on the composition thereof, there are photoreceptors having a positive electric and those having a negative electric. In the case of forming printing parts as electrostatic latent images by exposure, the images are developed by a toner of the opposite sign electrical charge. On the other hand, in the case of reversely developing printing parts by removing the electricity thereof, the images are developed by a toner of the same sign electrical charge. A toner comprises a binder resin, a coloring agent and other additives, and a charge control agent is usually used therein in order to provide desired frictional charge characteristics such as charge speed, charge level, and charge stability, temporal stability, and environmental stability. The charge control agent largely affects the characteristics of a toner.
Further, in the case of color toners, a light-colored and preferably colorless charge control agent is needed, which does not affect the hue. Examples of such light-colored or colorless charge control agents include metal complex salt compounds of hydroxybenzoic acid derivatives (see Patent Literatures 1 to 3), metal salt compounds of aromatic dicarboxylic acid (see Patent Literature 4), metal complex salt compounds of anthranilic acid derivatives (see Patent Literatures 5 and 6), organic boron compounds (see Patent Literatures 7 and 8), biphenol compounds (see Patent Literature 9), calyx(n)arene compounds (see Patent Literatures 10 to 16), and cyclic phenol sulfides (see Patent Literature 16) for a toner having a negative electric; and quaternary ammonium salt compounds (see Patent Literatures 17 to 19) for a toner having a positive electric.    Patent Literature 1: JP-B 55-042752    Patent Literature 2: JP-A 61-069073    Patent Literature 3: JP-A 61-221756    Patent Literature 4: JP-A 57-111541    Patent Literature 5: JP-A 61-141453    Patent Literature 6: JP-A 62-094856    Patent Literature 7: U.S. Pat. No. 4,767,688    Patent Literature 8: JP-A 1-306861    Patent Literature 9: JP-A 61-003149    Patent Literature 10: JP-B 2568675    Patent Literature 11: JP-B 2899038    Patent Literature 12: JP-B 3359657    Patent Literature 13: JP-B 3313871    Patent Literature 14: JP-B 3325730    Patent Literature 15: JP-A 2003-162100    Patent Literature 16: JP-A 2003-295522    Patent Literature 17: JP-A 57-119364    Patent Literature 18: JP-A 58-009154    Patent Literature 19: JP-A 58-098742    Patent Literature 20: JP-A 10-081680
However, many of these charge control agents are complexes or salts which comprise metals such as chromium and zinc, and not always safe since they have a problem with the waste regulations. In addition, such charge control agents are disadvantageous in that they can not be completely colorless; they are late in charging risetime; they have a problem with the environmental stability of the charge amount in hot and humid conditions; the charge amount thereof is low; oppositely-charged toners are numerously generated; or they are poor in dispersibility or stability of the compound. Thus, there has been no compound having satisfactory performance as a charge control agent.